1. Field of the Invention
This invention relates to building materials, and more particularly, to a building material substrate having an improved exterior finish and method for making the same.
2. Description of the Related Art
Fiber cement in recent years has become popular for use as a building material. In many instances, fiber cement is preferred over the more conventional materials such as wood, metal, or plastics. When compared with wood, fiber cement has better water resistance and is also less prone to rotting, cracking or splitting. Moreover, fiber cement does not rust like metal and is more durable to weathering than plastics. In particular, fiber cement products such as James Hardie Building Products"" HARDIPLANK(copyright) offer a lifetime of low maintenance and can be installed just as easily as wood sidings.
Advantageously, fiber cement can withstand extended exposure to humidity, rain, snow, salt air, and termites. It is also dimensionally stable and will not crack, rot, or delaminate under normal environmental conditions. Moreover, fiber cement panels may be pretextured or embossed to give the panel a desired look and feel. The panels may, for instance, be textured to resemble the look and warmth of natural wood. As such, fiber cement siding is a durable, attractive alternative to traditional wood composite, cedar, vinyl, brick or stucco sidings. Additionally, fiber cement is also an inexpensive alternative to conventional roofing materials such as corrugated aluminum sheets, which can be costly and energy intensive.
In some cases, the exterior surface of fiber cement panels is painted or subject to other types of post-production or on-site finishing to give the material the desired exterior appearance and feel for a particular application. Disadvantageously, however, natural weathering and other environmental factors can lead to chalking of the exposed paint surface and loss of polymer in the paint film. Moreover, the paint layers are typically very thin, generally on the order of one to two mils, and therefore are particularly susceptible to chipping, peeling, and scratching from surface abuse. Furthermore, the exterior surface of some building materials such as fiber cement can absorb up to about 30% water by weight, which may cause the panel to experience freeze-thaw during the winter and become damaged.
To improve the durability of the exterior surface of building materials, manufacturers sometimes bond protective films to the exposed surface so that it can better withstand exposure to the elements. The films can also be used to increase the aesthetics of the building material. One commonly used protective film is a polyvinyl fluoride (PVF) film manufactured by DuPont under the trademark TEDLAR(copyright), which has proven to be very durable for exterior applications. However, it is particularly difficult to bond TEDLAR(copyright) or other fluorohydrocarbon films to fiber cement substrates. The adhesives used to bond TEDLAR(copyright) films to building material substrates are generally undesirable, particularly for bonding the film to the fiber cement substrate.
Conventional adhesives typically do not provide durable adhesion between TEDLAR(copyright) and fiber cement because fluorohydrocarbon films such as TEDLAR(copyright) are generally not easy to wet and to form bonds with another surface. Moreover, fiber cement has weak surface layers that can be easily peeled off if the selected adhesive cannot strongly adhere to the fiber cement. Furthermore, these adhesives generally take an undesirably long time to set, thereby causing a reduction in production throughput. Additionally, the adhesives also contain volatile organic chemicals (VOCs) that have shown to cause adverse effects on the environment. As such, the exterior finish of fiber cement substrates are often left unprotected from the elements and can become damaged by environmental conditions.
Hence, from the foregoing, it will be appreciated that there is a need for a building material assembly having a durable exterior finish that is resistant to weathering. To this end, there is a particular need for a fiber cement assembly having a protective film bonded to the exterior surface of the fiber cement substrate so as to provide the assembly with a durable finish while maintaining a desired textured appearance. Furthermore, it will be appreciated that there is a need for an adhesive system that is adapted to form a durable bond between a fluorohydrocarbon film and a substrate surface, such as fiber cement.
The aforementioned needs are satisfied by the building material assembly of the preferred embodiments of the present invention. In one aspect, the building material assembly comprises a building material substrate, a fluorohydrocarbon film, and a layer of rapid-setting non-VOC adhesive positioned between the substrate and the film so as to form a durable bond therebetween. Preferably, the adhesive system comprises a one component polyurethane or polyurea adhesive composition including a reactive isocyanate compound and one or more catalysts. Preferably, the isocyanate compound wets the fluorohydrocarbon film and forms a physical bond with the film. Preferably, the catalyst catalyzes a reaction that forms a chemical bond between the isocyanate compound in the adhesive mixture and the hydroxyl functional groups in the substrate. In one embodiment, the adhesive system further comprises a plasticizer that modifies the rheological characteristics of the adhesive. In yet another embodiment, the adhesive system includes a defoamer surfactant that reduces the occurrence of blisters in the adhesive. Furthermore, the adhesive composition may also comprise additives such as antioxidant, moisture scavenger, UV absorber, and/or heat stabilizer to improve the durability of the adhesive.
In another embodiment, the adhesive system comprises a two component polyurethane adhesive composition including a reactive isocyanate compound, a polyol containing hydroxyl functional groups, one or more catalysts, and optionally a plasticizer, a defoamer surfactant, a moisture scavenger, an antioxidant, a UV absorber, and heat stabilizer. Preferably, the catalyst is adapted to catalyze a reaction between isocyanate and hydroxyl functional groups so as to form a polyurethane based polymer that will physically interlock and bond with the fluorohydrocarbon film and the substrate. In one embodiment, the polyol may be substituted by a polyamine so as to form a two component polyurea adhesive system.
Advantageously, the adhesive systems of the preferred embodiments of the present invention provide excellent adhesion for laminating a fluorohydrocarbon film to fiber cement as well as other substrates such as wood, metals, and plastics. Furthermore, the preferred adhesive systems do not contain any measurable amounts of volatile organic chemicals (VOCs) and therefore do not pose threats to the environment or worker""s health. The preferred adhesive systems also provide a fast working time and set time so as to increase the manufacturing throughput.
Furthermore, the building material substrate may comprise a fiber reinforced material, a metal material, a plastics material, or a wood material. The fluorohydrocarbon film preferably comprises a polyvinyl fluoride film such as TEDLAR(copyright) manufactured by DuPont. In one embodiment, the film is laminated to the substrate in a manner such that the texture and embossing on the substrate are transferred to the film. In another embodiment, the film is bonded to the exterior surface and side edges of the substrate so as to provide the substrate with a uniform exterior appearance.
In another aspect, the preferred embodiments of the present invention provide a method of manufacturing a building material assembly having a protective fluorohydrocarbon film bonded to the exterior surface of the substrate by a rapid-setting non-VOC polyurethane based adhesive. In one embodiment, the method comprises using a membrane vacuum press to laminate the film to the exterior surface and side edges of the substrate at the same time. In another embodiment, the method comprises using a continuous isobaric press to simultaneously laminate the film to the exterior surface and side edges of the substrate. Preferably, the continuous isobaric press includes a plurality of vertical rubber belts that are designed to fit in the gap between adjacent substrates and exert lateral pressure against the side edges of the substrates so as to press the film against the side edges while the pressure is also applied against the exterior surface of the substrate.
Advantageously, the building material assembly of preferred embodiments of the present invention and method of producing the same provide a building material having a durable exterior surface that can withstand weathering. Furthermore, the building material also retains its original aesthetic qualities as the texture and embossing on the substrate are fully transferred to the film. Furthermore, the side edges of the substrate are protected as well so as to inhibit UV damage that might otherwise damage the substrate. Furthermore, the method of producing the board significantly reduce the cycle time required as the exterior surface and side edges are laminated at the same time. These and other objects and advantageous of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings.